The present invention relates to a novel injection molding machine for molding microparts containing a plastic shot volume of between about 0.001 to 3.5 cubic centimeters. Specifically, the micro injection molding machine utilizes pneumatic cylinder or cylinders for the plasticization and delivery of the resin material to the injection portion of the molding machine. A linear motor drives the injection portion to inject the resin material through the nozzle into the mold cavity to complete the injection molding of the micropart.
Injection molding processes are well known and have been developed for molding plastic parts. These processes generally involve melting plastic or resin pellets by feeding the pellets through a heated screw barrel utilizing a rotating screw. The heated barrel together with the heat supplied by the shear of the plastic pellets heats the resin pellets above their melting point. The screw is supported axially with a load and as the molten plastic moves to the front of the screw, the buildup in pressure forces the screw backwards until a desired volume of plastic has been developed in front of the screw. At this point, the rotating screw is stopped and the molten plastic is injected by moving the screw forward to force plastic through the nozzle into the cooled mold cavity to provide the desired molded part. The mold cavity is cooled and the injected plastic is fixed to the desired shape of the part. Such known technology and operations require that the forward motion of the screw must fill the mold cavity to obtain a good quality, dense molded part.
The prior art processes for injection molding are adequate for molding normal size parts utilizing shot sizes in excess of 3.5 to 5.0 cubic centimeters; however, when the microparts require very small shot volumes of less than 3.5 cubic centimeters there are significant problems with existing processes and technology. For example, the screw or auger means used to transport the plastic or resin pellets must be miniaturized in diameter to accept the resin pellets. If the screw is too large, it will contain many volumes of plastic relative to the part being molded. In such a situation, the plastic remaining heated in the barrel after each molding cycle degrades over time when held at this melting temperature. However, if the screw or auger is miniaturized and the screw flight depths are smaller than the pellet size, problems exist concerning accepting the pellets and feeding the resin plastic or pellets into the auger to allow compression and melting of the plastic. Although resin pellet diameter sizes are normally in the range of 2.5 mm or greater, miniature pellets of about 1.25 mm exist. However, the screw injection processes are limited to injection moldings of shot sizes larger than 3.5 cubic centimeters, even when the plastic pellet size is about 1.25 mm.
Furthermore, it should be pointed out that the smallest available screw or auger today is 14 mm in diameter and such auger devices are unable to precisely meter and maintain the accuracy of the molded plastic below the resolution limit of the screw stroke injection machine.
Additionally, existing injection molding processes for molding microparts are unsatisfactory because the microparts often require a thin wall thickness ranging from about 0.025 to 0.30 mm. To force and inject the plastic into these thin walled microparts without freezing, very high pressures and short injection times are required. Existing conventional molding machines generate approximately 25,000 psi pressure and require a 0.5 second injection time for molding shot sizes greater than 3.5 cubic centimeters.
However, if it is desired to injection mold shot sizes or volumes containing less than about 3.5 cubic centimeters, the necessary force required approaches 100,000 psi and a 0.01 second injection time when the wall thicknesses of the micropart is approximately 0.05 mm. Thus, existing prior art molding machines and processes are incapable of molding plastic shot sizes or volumes approaching 3.5 cubic centimeters or less to provide uniform molded microparts without large variations in part dimensions from shot to shot.
Accordingly, to injection mold microparts the injection molding machine must create a high injection pressure and possess controlled injection speed profiles substantially less than 0.5 seconds. Also, existing technology and processes utilize hydraulic pressures to create the injection pressures and injection speed profiles. However, hydraulic fluids are not readily compatible with clean room facilities. Thus, the injection molding of medical grade devices and related microparts is severally limited with existing technology.
One attempt to overcome the problems of these known injection molding machines and processes, has suggested that the injection machine include a system wherein the heated plastic is plasticized and then introduced into the front of an injection plunger. However, such machines have poor quality control over the filling of the plastic into the mold cavity because they utilize or require air cylinders to drive the injection plunger, a structure and mechanism which cannot accurately control the speed of injection. More importantly, such injection molding machines cannot stop the injection process as the mold cavity is filled except by the increase in pressure buildup during the molding process. The control of the molding process by measuring the increase in pressure yields a high variability in the molded parts, a result which is unsatisfactory for most molded operations. U.S. Pat. No. 5,380,187 describes a molding machine comprised of a combination of a screw or auger to mix, heat and plasticize the plastic or resin material for deposit before an injection plunger to accomplish the filling process. However, such devices are limited to molding shot volumes of substantially greater than 3.5 cubic centimeters and are unsatisfactory for molding thin-walled microparts.